This invention relates to an improved heating system for a building structure utilizing two sources of heat.
With the advent of the energy crisis and the concommitant increase in the cost of energy, considerable efforts have been directed toward developing means and methods for conserving energy. In this regard, significant attention has been devoted to the revival of fireplaces for heating houses and other types of building structures. Some of these systems, such as that described in U.S. Pat. No. 549,071 of Aug. 11, 1925, utilize a fireplace to heat water for radiators in a house. Other systems, including the one described in Popular Mechanics (Oct. 1974, page 154 et seq), utilize a fireplace in conjunction with a standard oil or gas-fired furnace to heat water for the radiators. These systems can be operated on a cost saving basis compared to conventional systems primarily because of the lower cost of energy derived from wood fuel compared to the cost of energy derived from oil or gas. However, while advantageous from an economic standpoint, they are frequently disadvantageous from a standpoint of convenience, efficiency, control, safety and comfort.
It is not unusual for a house to become uncomfortably warm when using a fireplace to heat radiator water. Moreover, systems which are not properly installed or use improper equipment can pose substantial safety hazards in that high pressures and temperatures often build up within the system. In order to provide appropriate temperature control, some systems require the user to close a series of gate valves to stop water from flowing from the fireplace heater which in turn can cause undesirable temperature and pressure buildup of the residual water in the fireplace unit. While such systems result in a monetary savings, they are relatively inefficient.
The water heated by the fireplace often times is pumped through a non-operational furnace where a heat loss is encountered (because of the additional distance involved) prior to entering the radiators. This water is usually pumped by a continuously operated furnace pump which is designed to pump water heated by the furnace at a flow rate which is optimized for maximum heat transfer for the furnace water temperature and not for the fireplace water temperature. Thus, a pump thermally suitable for use with furnace water is generally not thermally suitable for fireplace water. A furnace pump is usually designed for intermittent operation responsive to a thermostat setting. It is designed to produce high flow rates for short periods of time in order to deliver a quantity of heat to a building to bring it back up to the thermostat setting as quickly as possible. Based on the heat transfer equation for water flow in a conduit, Q = W C.sub.p .DELTA. T (where Q is the heat transferred, W is the flow rate of the water, .DELTA. T is the temperature differential between the water entering and leaving the furnace and C.sub.p is a constant), it is apparent that when the furnace pump is not operational, the flow rate W of water is zero and there is no heat transferred. When the furnace pump is operational, W has a value and heat is transferred. Because of its intermittent operation, a furnace pump has a larger capacity than would be necessary if it were operated on a continuous basis for the delivery of a given quantity of heat. Obviously, unnecessary electrical energy is used when a high capacity furnace pump is used to continuously circulate fireplace water. Finally, most of the prior art systems are relatively inconvenient to operate. In addition to manually opening and closing gate valves, a user is required to turn the furnace off when the fireplace is in use by lowering the thermostat or resetting other controls or is required to shut the fireplace down when the furnace is in operation.